Huperzine A is a highly active alkaloid isolated from Melaleuca tower of Lycopodiaceae plant [Huperzia serrata (Thunb) Thev.]. Optical isomer (−)-huperzine A is usually employed as a pharmaceutically active ingredient. The structural formula of (−)-huperzine A is as follows:

The chemical name of (−)-huperzine A is: (5R,9R,11E)-5-amino-11-ethylidene-5,6,9,10-tetrahydro-7-methyl-5,9-methanocycloocta[β]pyridin-2-(IH)-one.
(−)-Huperzine A is a highly efficient and highly selective reversible inhibitor of acetylcholinesterase, can improve learning and memory efficiency, and can be used to treat a variety of neurological and psychiatric diseases.
(−)-Huperzine A tablets were launched in the Chinese market in 1995, and has been used for the treatment of Alzheimer's disease (AD) and memory disorders clinically. In other countries, huperzine A has been used as a food additive, and widely used as an active ingredient in functional drinks, with the purpose of improving the memory of the elderly and enhancing the reaction time of athletes. (−)-Huperzine A formulations can be used to improve learning and memory efficiency, and restore the functions of damaged neurons. It is mainly used for the treatment of myasthenia gravis, schizophrenia, dementia, benign memory impairment, etc.; in particular, amnesia and senile dementia, and effectively improving children's memories.
The amount of natural (−)-huperzine A in Melaleuca tower of Lycopodiaceae plant is merely about one in ten thousand, and the growing period of Melaleuca tower flora is up to 8-10 years. Simple extraction cannot meet the demand of the market; therefore, chemical synthesis is required in order to increase market supply.
The two strategies for chemical synthesis of (−)-huperzine A are asymmetric synthesis and racemic resolution method. Current asymmetric synthesis of (−)-huperzine A requires the use of expensive metal palladium catalyst and chiral ligands coordinated to palladium. The difficulties of recovering palladium catalyst, coupled with the high cost of preparation, isolation and purification of chiral ligands, form an obstacle to large-scale production of (−)-huperzine A. At present, such asymmetric synthesis can only be done in small laboratory scale, not suitable for large scale production, let alone providing the pharmaceutical industry with an industrial low-cost and convenient production of (−)-huperzine A.
Aside from asymmetric synthesis, the Patent CN101130520 reported a resolution approach to prepare (−)-huperzine A. Chang et al. first obtained racemic O-methyl-huperzine A by chemical synthesis, then formed diastereomeric salts between the racemic compound and an acidic resolving agent, (−)-2,3-dibenzoyl tartaric acid. Repeated recrystallization from organic solvent, followed by basifciation and deprotection, provided (−)-huperzine A. The reaction process is as follows:

In the patent of Chang et al., the yield of intermediate (−)-O-methyl huperzine A after resolution is 16%. Obviously, during the preparation of (−)-huperzine A, creative systematic research was not conducted on the resolution approach, resulting in about 34% of the final product being not obtained in the first attempt. Although the total yield can be improved after recovery, basification and repeated resolution, the process involved complex operation, and the quality of the recovered material is difficult to control. Therefore, the process does not meet the production requirement of GMP, and is also difficult to satisfy the API production requirement of the pharmaceutical industry.
Because the preparation route for (±)-O-methyl huperzine A is more than 10 steps, the overall yield of (±)-O-methyl huperzine A through these steps is very low (<5%). In addition, a number of expensive raw materials have to be used, such as the raw material acrylonitrile (>4000 Yuan per kilogram), methylation reagent Ag2CO3 (>6000 Yuan per kilogram), 30% Pd/C (>1000 Yuan per kilogram), lithium aluminum hydride, (>1900 Yuan per kilogram), etc. The production cost of (±)-O-methyl huperzine A is extremely high. If the yield of (±)-O-methyl huperzine A is calculated according to the reference [Chinese J. Med. Chem., 1992, 2(2), 1] in CN101130520, the production cost is more than 140,000 Yuan per kilogram. The detailed calculation is as follows:
PriceTotalRaw materialAmount/kg(Yuan/kg)cost/YuanAcrylonitrile4590040,500Ethyl acetoacetate9063.45,70630% Pd/C4.527,00012,150(recyclingrate 90%)Silver carbonate405,50022,000(recyclingrate 90%)Iodomethane5070035,000Lithium aluminum hydride6.51,95012,675Sodium cyanide156009,0002-methacrylaldehyde45002,000Diphenylphosphoryl azide12.04505,400Total~140000 Yuan
Based on the calculated cost as above, if (±)-O-methyl huperzine A is subject to chiral resolution based on the yield of 16%, the cost of the raw materials for (−)-O-methyl huperzine A will be increased from 280,000 Yuan/kg (a chiral resolution rate of 50% is used for calculation) to 870,000 Yuan/kg.
Since the chiral resolution yield has a decisive influence on the production cost for the final product (−)-huperzine A, it is very important to select proper procedures of chiral resolution. It is found by repeated experiments that the yield is relatively low via chiral resolution of (±)-O-methyl huperzine A and is difficult for further improvement. It is also found that (±)-huperzine A is rather poorly soluble in most solvents. This may be the reason why there has not been any report about chiral resolution in the last step.
The above experimental result indicates that direct chiral resolution of (±)-huperzine A is a highly challenging and creative task. After the inventors performed a number of screening investigations on resolving solvents, types of resolving agents, and resolving procedures, a method with an amazing >70% recycling rate of (−)-huperzine A was found by resolving a mixture of diastereomers: direct formation of a salt between (±)-huperzine A and a chiral acid in a proper solvent gives the (±)-huperzine A-chiral acid salt. (−)-Huperzine A with high purity is obtained after recrystallization and basification, and the yield of the target isomer is higher than 70%. Due to the increased yield, the production cost for (−)-huperzine A can be reduced to one half of that of current technology. In addition, the optical purity of the target product obtained by this method can be higher than 99.8%. The chemical purity can be higher than 99.5%. With further recrystallization, the purity can be higher than 99.9% and the amount of each impurity is lower than 0.02%. It fully meets the requirements for pharmaceutical raw materials.